High workload

Among the reasons given were the high workload in CID units, a lack of support for trainees and the intense scrutiny detectives face.

To help plug the gaps, some forces have begun recruiting specialists in cyber-crime and fraud to help with investigations.

Scotland Yard is now going a step further by allowing people to join as trainee detective constables, with full police powers.

Unlike other recruits, they will not have to spend any time in uniform doing street patrol and response work.

£30,000 salary

They will undergo an assessment involving tests, exercises, and interviews. If selected, they will undergo 18 weeks' training.

The training will be partly classroom-based, and partly based in boroughs. It will be similar to a constable's training, but with an investigative focus.

The initial salary will be just under £30,000.

The training will start in January. If successful, there will be another recruitment campaign during autumn.

David Tucker of the College of Policing, which sets standards of training and ethics for police in England and Wales, said: "We are aware of the difficulties in recruiting officers and staff into investigative roles, and we are working with colleagues, including the national policing lead, to support our members.

"The College and the NPCC are looking at the extent of this issue and we are currently working with a number of forces to develop a range of initiatives to address the problem."